Thousands of tadpoles are on a journey from Cleveland to Puerto Rico as part of an effort to save their critically endangered species.

The traveling tadpoles are Puerto Rican crested toads, and the Cleveland Metroparks Zoo is part of the Species Survival Plan that works to breed the toads and release them back to the wild. The plan is managed by the Association of Zoos & Aquariums.

"The Puerto Rican crested toad Species Survival Plan has been very successful," said Lynn Koscielny, the associate curator of animals at the zoo. "Field researchers in Puerto Rico have observed toads with transponders that were released into the wild returning to the protected ponds to reproduce."

The zoo already sent a group of toads to Puerto Rico in 2010, according to a zoo release, but the current group of 4,500 tadpoles is much larger.

The tadpoles will be sent to the U.S. Fish and Wildlife Service's Caribbean Refuge in San Juan and then taken to one of three potential release sites along the southern coast of Puerto Rico, which is an unincorporated territory of the United States.

The zoo has to breed the toads under conditions that mimic those they would have in their native environment.

"This involves cooling the toads down to 66 degrees (Fahrenheit —19 degrees Celsius) and then placing them in a rain chamber tank that simulates the rainy season in the toads' native Puerto Rico," the release said. "A sound recording of the male crested toad’s mating call adds to the simulated environment."